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Community and Q&A

2/12 cathedral roof replacement

Mark Kessler | Posted in GBA Pro Help on


I have a shingled 2/12 roof that needs to be replaced it is a cathedral ceiling. It has no dormers or right angles to it, the size is about 40′ by 20′. I am in Vermont so i have serious ice dams which i would like to minimize but heres the catch i am not looking for the 15k to 30k option. I am looking for the most reasonable balanced option under 10k. I have received a few quotes.

1. Strip roof, ice and water shield, architectual shingle ~5500k
– justification here is that the current shingled roof has about the lowest quality shingle on it now and has lasted maybe 10-15 years.
2. Strip roof, ice and water shield, standing seam with a 1″ high [email protected] ~6500k
3. Build new roof 5/12 common rafter, ice and water shield, shingle ~10k
– i think the bonus here would be that i would have the opportunity to create a cold roof and reduce the ice damming, the draw back is that quote assumes that i finish off some of the trim, painting and any insulation work.
4. Dense pack cellulose close off venting and rubber roof ~15k
– this one freaks me out for obvious reasons, I suppose it could be done properly but…

I am leaning towards #2 however it does nothing to solve the ice damming as far as i know, could you put it on purlins to assit in reducing the ice damming? Also if ice damming occurs can it get into the seam lock of the 1″ high seam?

As much as i would like to eliminate the ice damming i dont think i can afford it, i havent had any leaking in the house (Although i do see evidence of past leaks before my time here) in the past 5 years.

Any suggestions,ideas would be appreciated, thanks for your time Mark

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  1. GBA Editor
    Martin Holladay | | #1

    Ice dams are caused by air leaks and insufficient insulation, so addressing ice dams with a roofing-only solution is short-sighted. If you are planning to install new roofing, it's a golden opportunity to add a thick layer of rigid foam above your roof sheathing.

    Moreover, installing asphalt shingles on a 2/12 slope is problematic. (For more information on this topic, see Installing Asphalt Shingles on a Low-Slope Roof.)

    You haven't told us much about your existing insulation, so it's hard to give you good advice. I urge you to read the following article: Prevent Ice Dams With Air Sealing and Insulation.

    For information on insulating this type of roof, see How to Build an Insulated Cathedral Ceiling.

  2. Mark Kessler | | #2

    Hi, I am aware that air leaks and limited air flow are the cause of the ice damming. I have done the most that can be done with air sealing from the inside as possible whithout tearing down the drywall.

    The roof is 2x6 cathedral with 4" of the pink stuff, pretty pathetic.
    How much foam would it take?

    Again I am a bit short sighted on this because of the dollars involved and the remaining time that we will be in the house, at the most 10 years. I also would like to add that we heat with wood 100% currently we spend about 1k yr. So as bad as this sounds it doesn't cost much to heat.

    I understand that I am asking for a halfbaked solution on a site with pros so the chalange would be coming up with a balanced cost solution based on the above, if the only solution is spending 20k then I might have to live with the ice. Thanks mark

  3. Mark Kessler | | #3

    I had a few more thoughts.
    I know I said #4 above is crazy but I guess I just dont really understand how an unvented roof could work unless done perfectly I would be worried about mold/moisture but I guess its all about using the right amount of insulation to prevent that.

    what about pulling out the insulation through the soffits an filling the 6" cavity with spray foam? Anyways I guess I am open to the idea of unvented i just don't fully understand how robust it can be.

    All this also depends on finding a knowledgeable contractor that can do the work.

  4. GBA Editor
    Martin Holladay | | #4

    If you read the article I linked to in my first response (How to Build an Insulated Cathedral Ceiling), you'll find lots of information on unvented roof assemblies.

  5. Jonathan Rupp | | #5

    I also have a low slope shingled roof (but with foam insulation above the roof decking). The previous owners added on an addition where they used 2x4 sleepers (i think) to elevate the sheething over the insulation, creating a vent space under the shingles. At the same time, they added additional foam on the old part of the house, but didnt add the vent space. This weekend i was on the roof to check out how it was handling the 3' of snow and i found that I had large ice dams over the old, unvented portion of the house that needed clearing, but the vented addition (with the same insulation) had absolutely 0 ice dams.

    When I will have to repace my roof in the next ~5 years, my current plan is to add a few inches of xps to keep the current polyiso warm, and then using more 2x4 sleepers to make a vented metal roof across the entire roof.

    I kind of disagree with Martin in saying ice dams are caused by insufficent insulation and sealing... Although he is right if you have 2 inches of snow, but if you heat your house to a balmy 60 degrees, and it is 30 degrees outside, and you have 3' of snow, you would need a minimum of R500 to avoid ice dams.

    That said, for $800 retail, you could add another R10 of XPS on top of your roof deck (dont use polyiso as it performs poorly below 50F, or EPS as it absorbs water, shrinks, and falls apart when handled, negating the enviromental savings over XPS). Say that you loose $500 of heat through your roof per year, that could cut the losses by ~1/4 +? At 3-4% loan/investment cost, it should pay for itself in less than 10 years (not to mention cutting down on the cold trips to dig out and pick up more firewood under your 3' of snow).

  6. GBA Editor
    Martin Holladay | | #6

    I'm sorry if my words gave the wrong impression. I certainly agree with you that adding rigid foam above the existing roof sheathing, along with a "vented over-roof" above the new rigid foam, is an excellent solution.

  7. Jonathan Rupp | | #7

    I was just totally amazed in how well the vented over-roof performed vs. the unvented roof in all this depth of snow in Boston this year, and in a perfect controlled test (right next to eachother with nearly identical insulation) with lots of deep snow (as I hadnt removed any snow up until this weekend when the threat of rain spurred me into action). Also the ridge vents on the vented portion of the roof are under 3' of snow, yet it still performed.

    At the same time I am annoyed the previous owners went out of their way to add a few inches of polyiso during their remodel, but didnt add the vented assembly on what is a very simple roof without any valleys.

  8. Expert Member
    Dana Dorsett | | #8

    The membrane roof over the original roof deck should be sufficient air barrier- no need for spray foam in the rafter bays. If it's very regular spacing and easy to fit, R23 rock wool would be lower cost than spray foam or dense pack.

    In US climate zone 6 you need at least 50% of the total R above the roof deck to be able to protect the roof deck from wintertime interior moisture drives, so you need at LEAST R23, but it would take R26 to get you to a code-min R49. If you can find a local supplier for reclaimed roofing foam from commercial demolition it can usually be had for less than 1/3 the cost of virgin stock goods, (often cheaper than batts per R-foot.) A combination of 3" of 2lb polyiso (R5.5/inch nominal, for R16.5), with a 2.5-3" layer of EPS on top of that would get you there. The reason for two types of foam is that in cold climates the outer layers of polyiso will run colder than their high-performance zone, whereas EPS gains performance at lower temperatures.

    If you have a truck (or can borrow or rent one) you can buy this stuff in MA from multiple places. There are even a couple of well-stocked larger operations such as Nationwide Foam in Framingham, or Green Insulation Group in Oxford, MA but there are also smaller vendors (less well stocked- you may have to wait).

    With an R50-ish roof with thermal breaks over the rafters it takes a pretty deep snowpack to get into serious ice-dam territory, but if you have 2-6 feet of settled snow on the roof (like I had recently in Worcester, MA) you'll need to shovel it off anyway, for structural reasons.

  9. Keith Gustafson | | #9

    2/12, EPDM over foam

    You will get ice dams, but you won't care.

    Foam to your budget; looks to be about 500 bucks per inch for that size roof.

    I cannot see any other system being cheaper or more water tight

  10. Mark Kessler | | #10

    Thanks for all the responses, now that winter is coming to an end (and the 8" ice damming!) I have started to get quotes on a roof replacement. An option that came up that sounds like a good solution is below. does this sound like a good way to approach a roof replacement ?

    1. Strip the roof, shingles and decking and pull off the soffit material for reuse
    2. Air seal with spray foam, not the whole topside but top plates ridge area and other obvious places
    3. Construct a new roof at a pitch of 5/12 (or what ever makes sense) common rafter or premodern truss
    4. Roofing option
    A. Purlins with screw down metal roof
    B. Plywood decking with shingles
    C. Plywood with standing seam.
    6. Reinstall and add material for soffit and venting
    5. Insulate to r60 or more with blown in
    Is there anything I am missing here? Is the a better approach cost wise to the above?

    What I like about this is it gives me an opportunity to do some of the work easily saving some money I can do the air sealing, insulation and finish off the gabal end as far as the house wall siding is concerned
    It also increases the pitch of the roof giving the house a better look which is good for resale.

    I didn't originally want to spend more than 6-5k due to the fact that we will prolly never get pay back but after thinking about it more and over time I relized that if it cost 12-14k to do it right I am really only spending around 6k to have it done correctly, I have to replace no matter what so the 6k to do it wrong makes no sense.
    The other thing is that if I document the before and after well, when we go to sell the house it will make the asking price more solid.

    Thanks in advanced, mark

  11. Keith Gustafson | | #11

    seems massively expensive compared to adding some foam and roofing material

  12. Bob Irving | | #12

    I like the idea of adding a steeper roof for several reasons, but it needs to be done right, meaning you need to use raised heel trusses so that there is as much insulation at the eaves as everywhere else.

  13. Eric Habegger | | #13

    I agree with Keith, (reply 11). I think you are better off within the bounds of your original conception and budget. Since it does not seem like you are committed to living there past a certain point then what do you care about the angle of the roof? I'm sure there will buyers out there, just like you were, that the low sloping roof won't matter to.

    Lose the spray foam for sealing because it can be an environmental problem and can easily be done poorly due to shrinking and other issues. Spray foam is NOT a quick and effective solution for these kind of problems due to inherent installer problems in hard to reach areas. Spray foam has been an overly subscribed to solution by home improvement tv shows and is not the panacea people think it is.

    Instead use the foam over roof sheathing on the original 2/12 layer and use that layer, perhaps with epdm, to create the air and water seal. I should add that it does not seem necessary to create a cold roof via ventilation channels above the foam board for this type of roof in your climate. It might reduce ice damming. Then again, if you have enough rigid foam insulation it shouldn't be much of a problem. What you really want is that the heat in a cathedral ceiling does not escape. That's accomplished with the rigid foam and thorough air sealing of it.

  14. Mark Kessler | | #14

    Thanks for the responses, I have a few additional questions.

    Assuming I went with at least 6" of rigid foam on top of the stripped shingled roof ( or what ever it takes to get at least 1/2 recommended )

    If I went with rubber roof :

    1, Is there an issue with the wood stove pipe as far as embers or hot crud? Would it require extra protection of some sort around the chimney besides the normal flashing detail that would be used?

    2. Would you use ice and water shield on the whole roof deck under the foam, just 3 feet up or none?

    Assuming I went with at least 6" of rigid foam on top of the stripped shingled roof with plywood decking on top for a shingled or metal roof

    1. Would you use ice and water shield on the whole roof deck under the foam, just 3 feet up or none?

    This applies to both scenarios above

    1. The existing rafters are 8" I think, with 3.5" of the pink stuff.
    a. Do you leave it the way it is - vented?
    b. Close off the venting
    c. Remove the pink stuff, close off the venting and dense pack it?

    Again thanks for all the help!

  15. Keith Gustafson | | #15

    Hot embers:
    not a problem, the chimney should have enough height and EPDM is AFAIK no more flammable than shingles

    On my roof, yes I put ice and water on the deck just as if it was a shingle roof, 6 foot at edge, 3 foot at eaves, above and below all penetrations[2 skylights, the chimney] . Call me paranoid. Just keep it away from the EPDM as they are not compatible.

    Spec .06 EPDM, no vertical seams and real Grace ice and water. IF there are penetrations look up good details. Roofers seem to think that a seam at the bottom of a valley is just fine[ask me how I know]

  16. Mark Kessler | | #16

    Thanks Keith and Eric for setting me straight I have all these grandois ideas of making it all right but the budget and the payback don't work for me
    I do have some additional questions on the rubber roof details, I have read many articles on this site and others but still have not found answers.

    I have a quote coming my way for Edpm over 1/2" poly ISO, the roofer says this is his standard starting point for a low slope roof ( he said he won't even do anything else on this roof) he adds in an option for R10 and above based on my budget.
    He will close off the ridge vent.

    So from what I understand xps performs better than poly ISO at cold temps so I should go with that or a combo of poly ISO with xps on top.

    One thing that I cant seam to figure out is that if I add rigid foam on top of the deck, close off the ridge vent but still have the 4" of fiberglass in the rafter bay with the air space above it and the soffit vents still in place is there an issue with this (besides not being the optimal solution) will there be condensation issues? Will adding additional rigid foam above the 1/2" even do anything for keeping the house more warm and using less energy? Is the 1/2" even necessary. I just don't get how any insulation on the roof deck will increase the overall efficiency of the house with the air space and soffit vents still in place ( no ridge vent). the only thing I can think of is that with a low slope roof even with the ridge vent I am probobaly getting very little air flow anyways (not much chimney effect?)

    Thanks for your help, Mark

  17. GBA Editor
    Martin Holladay | | #17

    If you plan to install rigid foam above the roof deck, it is essential that you carefully seal the ventilation openings at the bottom of the roof (the soffit vents) as well as at the top (the ridge vents).

    Since you are in Vermont (Climate Zone 6), building codes require that the rigid foam layer that you plan to install above your roof sheathing must meet a minimum R-value -- namely R-25. If you use polyiso, that's about 4.5 inches. If you use XPS, that's 5 inches. If you use EPS, that's about 6 or 7 inches. For more information, see How to Build an Insulated Cathedral Ceiling.

  18. Keith Gustafson | | #18

    Why anyone is suggesting 1/2 iso foam is beyond me.

    Iso board is less than 60 cents a board foot[one foot by one foot by one inch thick] so your roof should cost an additional 500 bucks per inch. The additional labor should be minor.

    If he does not like it find another roofer

  19. Expert Member
    Dana Dorsett | | #19

    In Climate zone 6 derate exterior roofing polyiso to R5/inch and cross your fingers. It needs to be at LEAST as thick as an XPS solution to have the same thermal performance during the ice-damming season. A better solution would be 3' roofing iso (R15, derated) on the roof deck + 2" of EPS (R8) above that to keep the polyiso warm enough to be in a higher performance range.

    It's the R-ratio that's important for protecting the roof deck from interior moisture drives, and with 2x6 rafters the best you're going to do with batts is about R23 for fiber insulation between the rafters (using rock wool batts or dense-packed fiberglass), which means you can cheat the R25 prescriptive by R2-R3 without having your roof deck rot. That means 4.5" of XPS or polyiso would be enough for dew point control.

    An R45-sh roof with R22-25 continuous foam on the exterior won't completely cure the ice damming issue, and it DOES meet code-min if you calculate the U-factor. IRC 2012 requires U0.026 or less- see TABLE N1102.1.3: Assuming the rafters are 24" o.c. and not a lot of blocking the framing fraction is about 10%, which makes the 2x6/R23 + gypsum layer come in at about R19 after thermal bridging, and with R23 foam + 1/2" polywood roof deck + 1/2" OSB nailer deck the stuff above the rock wool/rafters layer comes in at about R24, for a whole assembly R of about R43, a U-factor of about U0.023, so it makes it with a bit of margin. If the rafters are 24" o.c. the framing fraction will run about 12%, which comes in at about U0.24, still makes it. If you have a bunch of blocking around penetrations and for racking forces and hit a 15% framing fraction (possible) you'll still make it, but barely.

    An R45-ish roof will improve the ice damming situation by a HUGE amount over 3.5-4" of mouse-eaten pink stuff, even if it doesn't cure it completely, and it CAN be done within budget if you use reclaimed foam rather than virgin stock.

    Assuming 5" of foam and a 20 x 40 roof that's 4000 board-feet of foam. If you buy used/reclaimed roofing foam from the bigger reclamation outfits in MA you can get R23-R25 for about $0.60-$1.00 per square foot, figure $1000-1200 material cost for the exterior foam if you include renting a box van to go pick it up. (The place in Oxford, MA is probably a bit closer to you than the place in Framingham, but you can figure that out. Either would be able to ship, for a price. See: )

    You can get R23 rock wool from box stores for about $1.15 per square foot , adding another ~$1K to the material cost. (You may be able to find it cheaper than that via building materials distributors.)

  20. Mark Kessler | | #20

    Dana , so If I add around $1200 for the foam on top then about 1k for the batts ( probably would have to do blown in or have the additional cost to remove and replace the deck) + close off the soffit vents + a rubber roof I am guessing the cost is around 8-10k. Is that correct? I will not be doing the work. If we were in this house for more than 5 years then it might make sense to spend the money but I am having a hard time justifying the cost when we only spend at the most 1k a year to heat the house, the house is comfortable and the current assembly does not leak and there is no sign of rot, I am inclined to put down exactly what is there now (shingles) or tpo and leave the ridge vent.

    Response #11 commented that my #10 seamed massive expensive to add a roof over of some sort compared to adding foam but the bottom line seams to be that it will cost 8-12k to do it right which there are many ways no matter what unless I just put on what I have now which was quoted for 5200k

    Am I missing something?

  21. Keith Gustafson | | #21


    I think you need iso on top to glue down the EPDM, the glue would melt polystyrene

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